Due to the decreasing availability and rising cost of cooling water, dry-cooling towers or air-cooled steam condensers are increasingly being used to dissipate heat into the environment in power plants that incorporate steam turbines. Unfortunately, at higher ambient temperatures, the effectiveness of such cooling systems decreases and this results in a corresponding reduction in turbine efficiency. Any such reduction in turbine output during hot periods may result in a significant loss in income, especially in areas where the demand and value of electrical energy during these periods is high. Not only that, but the capacity of the cooling system may need to be increased in order to cater for higher ambient temperatures thereby representing an increased capital cost and accompanying increased electricity production costs.
In a direct A-frame air-cooled steam condenser system of the type widely used in power plants, the steam exiting a steam turbine is fed via a steam header to a primary condenser arrangement and residual steam leaving the primary condenser arrangement is condensed in a dephlegmator that allows non-condensable gases to be separated out and typically vented to atmosphere. Such dephlegmators are available in either dry cooling or wet cooling varieties and for reasons set out above, dry cooling may often be preferred in spite of the disadvantage indicated.
As an alternative to the above, so-called adiabatic arrangements have been used in which water is sprayed into the inlet air. However, the improvement achieved in this instance does not appear to be commensurate with the cost involved. However, for the nozzles tested only between 60% and 70% of the spray water is evaporated and even the introduction of a drift eliminator cannot ensure that the finned surfaces of the heat exchange tubes remain dry. The unevaporated water droplets that accumulate on the structure can lead to corrosion of the structure surfaces as well as undesirable rainback that may cause surface and ground water contamination. Finer sprays may be achieved with smaller high pressure nozzles. Due to practical and cost considerations spray cooling of inlet air does not appear likely to find application in large air-cooled condensers.
There is thus a need for a dephlegmator that is aimed at providing an improvement to the existing situation.